If the total amount of adenine and thymine in a double-stranded DNA is 45%, the amount of guanine in this DNA will be

The correct option is B 27.5%
In a DNA segment, the amount of adenine is equal to that of thymine. According to given information, adenine and thymine constitute 45% of DNA. So, guanine and cytosine will constitute 55%.

Amount of guanine is equal to cytosine. So, amount of guanine in DNA will be 55/2 = 27.5%.